Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Detail) - JPY (¥) ¥ in Millions		Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		¥ 234,117	¥ 1,048,062
Derivative assets		10,486	39,526
Derivative liabilities		32,822	35,724
Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		4,688
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		234,117	1,048,062
Other investments	[1]	13,914	20,395
Derivative assets	[2]	10,486	39,526
Total assets		4,079,047	3,455,847
Derivative liabilities		32,822	35,724
Total liabilities		32,822	35,724
Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,643,589	1,409,610
Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		67,497	67,569
Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		219,388	208,708
Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[3]	161,495	69,539
Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	360,867	366,465
Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	190,112	99,350
Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,172,894	126,623
Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		4,688
Marketable securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		234,117	1,048,062
Total assets		1,318,251	1,173,352
Marketable securities | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		18,719	20,473
Marketable securities | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7,768	8,548
Marketable securities | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		11,472	8,041
Marketable securities | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[3]	3,984
Marketable securities | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	90,801	88,228
Marketable securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		951,390
Securities investments and other | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	[1]	13,914	20,395
Total assets		2,750,310	2,242,969
Securities investments and other | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,624,870	1,389,137
Securities investments and other | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		59,729	59,021
Securities investments and other | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		207,916	200,667
Securities investments and other | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[3]	157,511	69,539
Securities investments and other | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	270,066	278,237
Securities investments and other | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	190,112	99,350
Securities investments and other | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		221,504	126,623
Securities investments and other | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		4,688
Other current assets/liabilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	[2]	9,431	37,003
Total assets		9,431	37,003
Derivative liabilities		19,852	20,550
Total liabilities		19,852	20,550
Other noncurrent assets/liabilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	[2]	1,055	2,523
Total assets		1,055	2,523
Derivative liabilities		12,970	15,174
Total liabilities		12,970	15,174
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		22,105	712,113
Other investments	[1]	5,489	6,192
Derivative assets	[2]	444	2,194
Total assets		1,065,138	846,829
Derivative liabilities		136	1,407
Total liabilities		136	1,407
Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,037,100	126,330
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		212,012	335,949
Other investments	[1]	1,507	5,099
Derivative assets	[2]	10,042	37,332
Total assets		2,819,204	2,488,422
Derivative liabilities		32,686	34,317
Total liabilities		32,686	34,317
Level 2 | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,643,589	1,409,610
Level 2 | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		67,497	67,569
Level 2 | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		219,388	208,708
Level 2 | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[3]	161,495	69,539
Level 2 | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	338,163	338,587
Level 2 | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	25,029	15,736
Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		135,794	293
Level 2 | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		4,688
Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	[1]	6,918	9,104
Total assets		194,705	120,596
Level 3 | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	22,704	27,878
Level 3 | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	¥ 165,083	¥ 83,614

[1] Other investments include certain hybrid financial instruments and certain private equity investments.

[2] Derivative assets and liabilities are recognized and disclosed on a gross basis.

[3] 2,875 million yen and 4,910 million yen are included in foreign securities for which the fair value option has been elected and classified in level 2 for the fiscal years ended March 31, 2018 and 2019, respectively, and are included in the consolidated balance sheets as securities investments and other.

[4] 160,470 million yen and 173,964 million yen are included in foreign securities for which the fair value option has been elected and classified in level 2 for the fiscal years ended March 31, 2018 and 2019, respectively. In the consolidated balance sheets, 25,955 million yen and 33,391 million yen are included as marketable securities and 134,515 million yen and 140,573 million yen are included as securities investment and other for the fiscal years ended March 31, 2018 and 2019, respectively.

[5] 93,971 million yen and 185,195 million yen are included in foreign securities for which the fair value option has been elected and classified in level 2 and level 3 for the fiscal years ended March 31, 2018 and 2019, respectively, and are included in the consolidated balance sheets as securities investments and other.